1. Field of the Invention
The present invention relates to an imaging apparatus, and more particularly to an imaging apparatus in which white balance is correctly adjusted after the type of illumination light source has been discriminated.
2. Description of the Related Arts
An imaging apparatus including a digital still camera and an image input device is provided with a mechanism for adjusting white balance based on photographing under different illumination light sources having individual spectral characteristics, such as a tungsten light source, a fluorescent light, and daylight.
A white object photographed under illumination by arbitrary light source becomes achromatic. The white balance is adjusted by setting gain of R, G and B. For example, in the case of the digital still camera, it is common to adjust the white balance by setting the gain of R, G and B according to the spectral characteristics pattern, which is selected as the most approximate pattern to the predetermined pattern of each illumination light source including the tungsten light source, the fluorescent light and the daylight, after estimating roughly the spectral characteristics of the illumination light source by comparing the output level of the image signals of R, G and B taken out from the image sensor.
There has been known an image sensor having plural pixels in which each pixel has a main pixel and a sub pixel. The main pixel has a relatively large area, and the sub pixel has a relatively small area. It is advantageous in such an image sensor in terms of obtaining a high-quality photographic image by applying an interpolation process to the image signal from the main pixel by use of the image signal from the sub pixel after taking image signals from the main and the sub pixels. For example, if sensitivity difference between the main pixel and the sub pixel is utilized, it becomes possible to perform the interpolation process such as widening a dynamic range.
However, in the prior method for estimating the spectral characteristics from the output levels of image signals of R, G and B output from the image sensor, although it is possible to distinguish roughly the type of light source such as the tungsten light source and the fluorescent light, it is difficult to discriminate in detail the difference in the spectral characteristics based on the type of fluorescent light. Especially, since the spectral characteristics in each fluorescent light is varied according to each product, the predetermined spectral characteristics pattern and the spectral characteristics of the fluorescent light used as the illumination light source in the photographing are different. Therefore, there arises a problem in that the white balance cannot be adjusted correctly.
Additionally, in the imaging apparatus using the image sensor including the main pixel having a relatively large area and the sub pixel having a relatively small area, the discrimination accuracy for the illumination light source is the same as the case of an imaging apparatus using the image sensor including a single photosensitive portion since the main and the sub pixels according to the conventional imaging apparatus have the same spectral sensitivity characteristics.